As a conventional electrolytic solution for operating an electrolytic capacitor, in particular, as an electrolytic solution for a medium-to-high voltage electrolytic capacitor, solutions obtained by adding boric acid or ammonium pentaborate serving as an electrolyte to a solution comprising ethylene glycol as a main component have been used. In an electrolytic solution containing boric acid, the boric acid is reacted with ethylene glycol so that a large amount of water is produced by condensation, which results in increasing water content in the electrolytic solution system. Therefore, when an electrolytic solution containing boric acid is used at a temperature of more than 100° C., the water in the electrolytic solution is evaporated to form a water vapor. Thus, the internal pressure in the package of the electrolytic capacitor is increased, and the electrolytic capacitor tends to be damaged. In order to overcome such a disadvantage, an electrolytic solution containing linear saturated dibasic acid or a salt thereof has come to be used.
However, since the solubility of linear saturated dibasic acid with respect to a solvent such as ethylene glycol is low, the linear saturated dibasic acid tends to be precipitated in the form of crystals at a low temperature, and therefore the problems that overcurrent is generated, and the low temperature characteristics of the capacitor deteriorate cannot be avoided.
Therefore, in recent years, it is attempted to use branched saturated or unsaturated dibasic acids, for example, dibasic acids such as 7-methyl-tetradecane-1,14-dicarboxylic acid, 7,12-dimethyl-octadecane-1,18-dicarboxylic acid, 7,12-dimethyl-7,11-octadecadiene-1,18-dicarboxylic acid, 7-methyl-7-tetradecene-1,14-dicarboxylic acid (Japanese Laid-Open Patent Publication No. 57-27013, Japanese Patent Publication Nos. 63-54209, 62-11765, 62-8005, and 62-7684, Japanese Laid-Open Patent Publication Nos. 62-254416, and 64-31408), 7-vinyl-9-hexadecene-1,16-dicarboxylic acid (Japanese Laid-Open Patent Publication No. 4-186713) and their salts as a solute.
The solubility of such branched unsaturated dibasic acids or their salts in a solvent such as ethylene glycol has been improved from that of linear saturated dibasic acids as described above or their salts. Therefore, the branched unsaturated dibasic acids or their salts are useful as a solute of an electrolytic solution. In particular, a dibasic acid of 7-vinyl-9-hexadecene-1,16-dicaroboxylic acid or its salt is useful as a solute of a specific electrolytic solution having electrical characteristics having a high breakdown voltage that exceeds a medium voltage region. This seems to be because the vinyl group in position 7 has a high ability of forming a complex with an aluminum electrode foil.
However, when the branched unsaturated dibasic acids as described above or their salts are used as a solute of an electrolytic solution for a high voltage capacitor, the electric conductivity (mS/cm) or the sparking threshold voltage (Vsp) of the electrolytic solution is reduced in use over a long time, and the water content tends to be increased. Therefore, the performance as a high voltage capacitor is not always adequate.
Therefore, there is a demand for a composition for an electrolytic capacitor that can be used for a high voltage capacitor, does not allow the electric conductivity (mS/cm) or the sparking threshold voltage (Vsp) of the electrolytic solution to be reduced in a long time use and prevents the water content from increasing, in particular, a composition for a high voltage electrolytic capacitor.